There is a general relationship between the energy and wavelength of a photon. A semiconductor material having an energy band gap Eg (eV) is sensitive to the light of wavelength .lambda. (.mu.m), EQU .lambda.=1.24/Eg.
Hg.sub.0.8 Cd.sub.0.2 Te has an energy band gap of about 0.1 eV and has been employed as semiconductor material for absorbing infrared rays 8 to 12 microns in wavelength. However, this material vaporizes at about 100.degree. C. because Hg is volatile and has a high vapor pressure. Therefore, handling Hg in thin film and wafer processing is difficult and has been an obstruction to development. This results in such difficulty in production that the number of picture elements in an imaging device is limited.
On the other hand, research on III-IV materials, such as GaAs, is advancing. An infrared detector element having a sensitivity comparable to HgCdTe uses multi-quantum wells (hereinafter referred to as MQW).
FIG. 8 is a cross-sectional view showing an MQW infrared detector element described in Applied Physics Letters, Volume 50, Number 16, April 1987, pages 1092-1094. In FIG. 8, reference numeral 1 designates a semi-insulating GaAs substrate. An n.sup.+-GaAs contact layer 2 is epitaxially grown on semi-insulating GaAs substrate 1 to a thickness of about 1 micron. Reference numeral 3 designates an AlGaAs-GaAs MQW structure. This AlGaAs-GaAs MQW structure 3 comprises fifty individual quantum wells, each comprising two Al.sub.0.31 Ga.sub.0.69 As barrier layers 3b 300 angstroms thick and a GaAs quantum well layer 3a 40 angstroms thick sandwiched by the barrier layers 3b. The total thickness is 1.73 microns. This MQW structure is produced on the n.sup.+ type GaAs layer 2 by MOCVD or MBE. An n.sup.+ -GaAs contact layer 4 is disposed on the AlGaAs-GaAs MQW structure 3. Reference numerals 12 and 14 designate electrodes. Reference numeral 21 designates an infrared entrance window which is produced by polishing a face of a semi-insulating GaAs substrate at an angle of 45 degrees. Reference numeral 31 designates infrared rays 8 to 12 microns in wavelength. Reference numeral 27 designates an infrared detector element.
In the ground state of n=1, the electron distribution in the quantum wells has a peak at the center of the GaAs quantum well layer 3a. On the other hand, in the excited state of n=2, the electron distribution has a peak in the neighborhood of the interface between the GaAs quantum well structure 3a and the AlGaAs barrier layer 3b, thereby producing a dipole moment perpendicular to the layers due to an optical transition. Of the electric field components present in electromagnetic waves, e.g., infrared rays, only the electric field component parallel to the dipole moment interacts with the dipole moment. However, the electric field of the infrared rays that are incident perpendicular on the MQW structure 3 is shifted perpendicular to the incident direction, that is, parallel to the MQW layer 3. Therefore, the electric field does not interact with the dipole moment and there is no light absorption at the MQW layer 3.
In order to absorb incident light at the MQW layer 3, the infrared rays must be incident on the MQW layer 3 parallel or oblique to the layers. However, in order for the infrared rays to be incident on the MQW layer 3 parallel to the layer, the light has to be incident from the substrate side. This requirement obstructs integration with other elements. In the element of FIG. 8, the face of substrate 1 is polished at an angle of 45 degrees, and the infrared rays 31 are obliquely incident on the MQW layer 3 from the window 21.
While the refractive index of air n.sub.1 is 1, the refractive index of semiconductor n.sub.2 is quite high, about 4. Therefore, the infrared rays 31 incident on the entrance window 21 in all directions are incident approximately perpendicular to the polished surface due to the refraction of the rays. The infrared rays 31 are incident on the MQW layer 3 at an angle of 45 degrees, and about half of the incident light has an electric field component parallel to the dipole moment of MQW layer 3 whereby about half of the incident light is absorbed.
FIG. 9 shows an energy band diagram of an AlGaAs-GaAs MQW structure 3. Herein, an electric field is applied to the MQW structure 3. Electrons are locally present in the neighborhood of the GaAs quantum well 3a and the energies of electrons are quantized, i.e., the electrons have discrete energies. The quantized energies are obtained from the following formula with reference to the bottom of the GaAs conduction band: EQU En=(h.sup.2 /2m*) (n.pi./L.sub.w).sup.2
where n is an integer (n&gt;1), h is Planck's constant, m* is the effective mass of the electron, and L.sub.w is the thickness of the GaAs quantum well layer 3a.
Herein, n=1 represents the ground state and n=2 represents an excited state. The thickness L.sub.w of the GaAs quantum well layer is selected so that the excited state n=2 is equal to the conduction band level of the AlGaAs layer 3b. The energy .DELTA.E required for excitation from the state n=1 to n=2 is about 0.1 eV, that is, the optical transition from the state of n=1 to n=2 is caused by the incidence of infrared rays 10 microns in wavelength, and infrared rays corresponding to the optical transition are absorbed. The excited electrons are able to move in the AlGaAs-GaAs MQW structure because of the electric field and contribute to the electrical conductivity. The infrared detector element of FIG. 8 is a photoconducting type, changing conductivity due to infrared ray absorption.
FIG. 10 shows a bias circuit for detecting a change in the resistance of the infrared detector element of FIG. 8. As described above, the infrared detector element 27 is a photoconducting type, and its resistance changes due to the incidence of light. Therefore, the change in the voltage between the terminals 30a and 30b, where a resistor 28 has a high resistance relative to the element 27 and is connected in series with the voltage power supply 29 and a constant current flows, is a measure of the amount of light which is incident on the infrared detector element 27.
In the prior art MQW infrared detector element of such a construction, in order to absorb light, the infrared rays 31 are required to be incident on the infrared entrance window 21 which is produced by polishing the substrate 1 in a diagonal direction. The infrared rays cannot be incident on the main, i.e., front, surface or rear surface of the semiconductor. Therefore, it is quite difficult to integrate a plurality of infrared detector elements. Furthermore, since the prior art infrared detector element is a photoconducting type, a bias circuit is required for detecting incident infrared rays, complicating the circuitry. Therefore, such an infrared detector element is inconvenient.